Electrical appliances having mains filters and different power consumptions are widely known. Particularly information technology and consumer electronics appliances often have not only a normal, fully switched-on mode of operation, but also what is known as a standby mode of operation in which only a relatively low power is drawn from a power supply system. Such standby modes of operation allow, inter alia, electrical appliances to be put into a fully switched-on operating state again under remote or time control.
A drawback of such electrical appliances is that they always draw a low power from the power supply system even in the standby mode of operation. Furthermore, power supply units and their components used to supply power to the electrical appliance usually exhibit lower efficiency in the standby mode of operation than in the power range which is designed for normal operation.
An example of such a component is mains filters and circuits for power factor correction used particularly when powerful switched-mode power supply units are used to reduce voltage and current spikes which occur therein.
FIG. 2 shows a conventional arrangement comprising an input circuit 1 with a mains filter for an electrical appliance. The circuit arrangement 1 comprises a mains input 3 in the form of a phase input Line and a neutral conductor Neutral coupled to a rectifier circuit in the form of a bridge rectifier BD 1 via a mains input filter. The mains input filter comprises X capacitors Cx1 and Cx2 arranged between the phase input Line and the neutral conductor Neutral, y capacitors Cy1, Cy2, Cy3 and Cy4 arranged between the phase connection Line or the neutral conductor Neutral and electrical ground, interference suppression coils L1 and L2 arranged in the phase line Line and the neutral conductor Neutral, respectively, and a discharge resistor Rx connected between the phase line Line and the neutral conductor Neutral. The bridge rectifier BD1 comprises four diodes arranged in what is known as a “Graetz bridge” and which convert an AC voltage on the connections 4 and 3 into a pulsating DC voltage on the connections 1 and 2. The actual power supply unit is not shown in FIG. 2. In the circuit arrangement 1 in FIG. 2, it has been connected in parallel with the storage capacitor denoted by Cb.
To avoid a large charging current when the power supply unit is switched on by the switch Sw in the phase line Line, the rectifier BD1 and the storage capacitor Cb have a current limiting element connected between them in the form of an NTC thermistor Rntc. The NTC thermistor limits the charging current in the capacitor Cb when the mains cable is switched on or plugged in. To avoid the parasitic load of the NTC thermistor Rntc during operation of the circuit arrangement 1, a monostable relay Rel is provided. By applying a voltage of twelve volts, for example, between the control connections A and B of the relay Rel, it is possible to short the current limiting element Rntc.
A drawback of the circuit arrangement 1 shown in FIG. 2 is that the mains input filter and storage capacitor Cb are always connected to the power supply system when the switch Sw is closed. Even if a power supply unit were therefore to draw no charge from the storage capacitor Cb, the mains input filter would result in an apparent power and power loss through the circuit arrangement 1. The discharge resistor Rx in the mains input filter also contributes to the power loss in the circuit arrangement 1. It is required for safety reasons to discharge the mains-side X capacitors Cx1 and Cx2 in the mains input filter which have a capacitance of more than 100 nF, for example, in a controlled fashion upon isolation from the mains. According to relevant specifications, for example, the VDE standard “Einrichtungen der Informationstechnik—Sicherheit” [Information technology devices—safety] DIN EN 60950-1, X capacitors in mains filters must be discharged to less than 37% of the starting value within a time of less than one second. Alternatively, the total capacitance of all X capacitors must be limited to a value of no more than 100 nF. This results in the efficiency of an electrical appliance being reduced particularly in a mode of operation with reduced power consumption.
It could therefore be helpful to provide an input circuit for an electrical appliance which improves the efficiency of the electrical appliance in an operating state with a reduced power consumption. In particular, it could be helpful to reduce the power loss from electrical mains filters or circuits for power factor correction present in the input circuit.